Pure gases are a critical component in many industrial processes today. Gas purity is typically measured as the quantity of all contaminants as a percentage of the total, e.g., 99.9999% (referred to as "6 nines pure"), or the quantity of individual impurities in ppm (parts per million) or ppb (parts per billion).
Industry requirements for the purity of gases are continually increasing. Many gases are now required to be "8 or 9 nines" pure. Such purity requirements are very difficult to achieve in the initial purification process, and they are almost impossible to maintain to the point of use after the gas has been put into a gas cylinder, and the cylinder has been shipped to the customer and attached to an appropriate gas regulator and gas piping. The gas cylinder, regulator and piping are all sources of contamination.
One way to provide gases of required purity at the point of use is to perform a final purification process immediately before use. An efficient and cost-effective purification process at the point of use involves passing the gas through a chemically reactive resin bed that chemically absorbs certain impurities, such as oxygen, water vapor or hydrocarbons. The resins are typically placed in a stainless steel cylinder which has appropriate pipe fittings such that the cylinder containing the resin can be placed in the gas line for a final purification.
One problem associated with the use of a resin is determining when the resin has become chemically depleted and no longer able to purify the gas. A solution is to install equipment such as residual gas analyzers, spectrometers or other electronic instruments to measure gas purity at the point of use. In most cases, however, such a solution is prohibitively expensive.
Another solution involves observing some physical change that occurs in certain resins when they become chemically depleted. For example, Saes Corporation manufactures a resin whose electrical conductivity changes as the resin becomes chemically depleted. A purification module includes the resin in a stainless steel container, with appropriate pipe fittings and a resistance gauge to determine when the resin is depleted. U.S. Pat. No. 4,782,226 describes a gas purification module containing a resin whose reflectivity changes as the resin is depleted. The reflectivity is monitored and the purification module is replaced when the reflectivity measurements indicate that the resin has become depleted.
There are several weaknesses with both of these methods. Most resins which are used for gas purification purposes are not electrically conductive. Furthermore, using electrical conductivity to measure resin depletion requires an electrical connection to the resin. Many gases requiring purification are extremely corrosive and it is therefore very difficult to maintain a good electrical connection in such an environment. Also, many resins do not exhibit a change in reflectivity with resin depletion or exhibit such a small change in reflectivity that detecting the change requires very sensitive instruments.
Accordingly, it is an object of the present invention to provide a means of determining resin depletion that does not rely on the conductivity or reflectivity of the resin.